The invention relates to platelet-shaped metallic effect pigments with a hybrid inorganic/organic layer, and to a method of producing them. The invention further relates to the use of these metallic effect pigments. The optical effect of metallic effect pigments is based on their platelet-shaped (or lamellar) structure, which leads to an orientation substantially parallel to the substrate in an application medium. The platelet-shaped structure of the oriented metallic effect pigments in turn produces effect-imparting properties such as, for example, a light/dark flop and also a high gloss.
The specific optical effect is very importantly determined by the pigment size and pigment size distribution and also by the average thickness of the metallic effect pigments. In terms of their platelet-shaped structure, metallic effect pigments are sensitive to the influence of mechanical forces, and particularly of shearing forces. By excessive shearing operations they may become disintegrated or deformed, thereby impairing the effect-imparting properties. The sensitivity of the metallic effect pigments is manifested, for example, in the fact that the metallic effect pigments are easily damaged or disintegrated on pumping through the circuit line systems of automobile finishing equipment. Additionally, in contrast to conventional color pigments, metallic effect pigments must be incorporated gently into wet coating materials, in order to avoid damage to the metallic effect pigments. Furthermore, metallic effect pigments cannot be incorporated, like chromatic pigments, by extrusion and subsequent grinding in a pinned disk mill, into the base varnish of a powder coating material. In that case they would generally be comminuted to such an extent that the characteristic optical effects would be lost almost completely.
A further critical point relates to the corrosion resistance of metallic effect pigments. Mention may be made here, for example, of the gassing stability of aluminum pigments in alkaline aqueous varnishes. In these media, unprotected aluminum pigments undergo corrosion, giving off hydrogen, a phenomenon also referred to as “gassing”. This unwanted event is associated with a potential for explosion, and, furthermore, oxidation robs the aluminum pigments of their typical optical properties. Furthermore, the hydrogen produced adversely affects the rheological properties of the coating material.
For a relatively long time, therefore, metallic effect pigments have been provided with purely inorganic or with purely organic three-dimensionally crosslinked coatings. These coatings generally serve as a protection against corrosion with respect to aggressive media, but often also have a mechanically stabilizing action. Moreover, metal pigments which are used in the powder coating material may be given a suitable electrostatic chargeability by means of dielectric coatings.
Metal pigments can be coated, for example, with silicon dioxide (U.S. Pat. No. 2,885,366 and U.S. Pat. No. 3,954,496) or with acrylate polymers (DE 40 30 727). SiO2-coated metal pigments are available commercially and are sold by Eckart GmbH & Co. KG under the names PCR, Hydrolan®, and also Resist and Dorolan®. Silicon dioxide coats endow aluminum pigments, for example, with excellent gassing stability in aqueous varnish systems. Moreover, the hardness of such a coating stabilizes the ductile and shear-sensitive aluminum flakes against the influence of shearing forces of the kind which occur, for example, in the circuit line systems in automobile finishing equipment (A. Kiehl and K. Greiwe, Progress in Organic Coatings 37 (1999) 179).
A similar effect also occurs in the case of aluminum oxide coatings (DE 195 20 312, H. Birner and K. Greiwe, Coating 11 (1997) 432). Also known are chromated aluminum pigments (EP 0 259 592), where an impervious mixed layer of aluminum oxide and chromium oxide provides for gassing stability.
The advantageous properties achieved through purely inorganic or purely organic three-dimensionally crosslinked coatings on metallic effect pigments must be distinguished from different surface coverings on the pigments. The aim of such surface coverings is always to improve the performance properties, which are influenced by the surface chemistry of the metallic effect pigments. Thus, for example, the wetting of the metal pigments is influenced directly by the surrounding varnish medium. Thus DE 198 20 112 A1 describes reactive organic orientation assistants which are able to attach chemically to a functional group on the surface of a metallic effect pigment and to attach to another functional group on the varnish. The organic orientation assistants are applied as a separate coat to metallic effect pigments which have been given inorganic oxide coats or organic polymer coats. The orientation assistants alter the surface properties of the metallic effect pigment and allow covalent attachment to the binder of the varnish, thereby improving on the one hand the orientation of the pigments in the varnish and on the other hand the condensation resistance of the cured varnish.
DE 196 35 085 A1 discloses aluminum pigments coated with a passivating protective coat and produced by Physical Vapor Deposition (PVD). A protective layer of inorganic oxides and organic oligomers and/or polymers bonded covalently to one another is not described.
DE 40 30 727 A1 contains resin-coated metal pigments which on their surface first have a covalently bonded siloxane layer to which a three-dimensionally crosslinked synthetic resin coating is bonded covalently. A disadvantage of these pigments is that they are not very stable toward corrosion. Additionally, this siloxane layer does not produce effective mechanical attachment of the synthetic resin coating to the metal pigments.
EP 1 322 714 A2 discloses a pigment preparation containing metal pigments coated with a silicon-oxygen matrix. This refers to pure SiO2 coatings and also to coatings with an SiO2 matrix into which organofunctional silanes have been incorporated.
WO 03/014228 A1 discloses metal pigments which have been coated with a first coat of phosphates or borates and with a second coat of SiO2. According to the teaching of that specification the SiO2 layer may also comprise organofunctional silanes.
A disadvantage of metallic effect pigments with a purely inorganic coating, or of inorganic coats into which organosilanes have been incorporated, is that these coats are very brittle. It has emerged that, under severe mechanical stress, these coats may be damaged, leading to a loss of desired properties.
Thus, for example, the processing of silicate-coated metallic effect pigments in a mixer can lead to a loss of gassing stability on the part of the pigments. In such an operation the pigments as a powder are pasted with solvent in the mixer, for example. These pastes possess extremely high viscosities. As a result, the mechanical shearing energy of the mixer blades imposes strong shearing forces on the metallic effect pigments. The brittle SiO2 coats may be mechanically broken by the shearing forces which act. Thus, in certain circumstances, injury to the coating of only a tiny fraction of the platelets is enough to lead to a reaction with surrounding water molecules, which first entails an explosion risk and second leads to a deterioration in the optical properties as a result of corrosion, such a deterioration being unwanted. For the production of metallic effect pigments in consistent quality, however, processing in a mixer is an unavoidable production step.
Quality detractions of this kind are the case even when the substrate used is an iron oxide-coated aluminum pigment (“Paliocrom”). These pigments can be made very stable to gassing, by means of a further SiO2 coating. In this case there are two successive inorganic coats, but they are both brittle.
The problem of brittleness cannot be solved simply by increasing the thickness of the SiO2 coat, or by similar measures, without having to accept significant deteriorations in other performance properties. Increasing the coat thickness impairs the pigment's opacity and leads to an increasing deterioration in the optical properties of the metal pigment. Nor does an additional organic functionalization of the surface produce any change in the fundamental brittleness of the SiO2 coat.
A disadvantage of the purely organic coating of metallic effect pigments is that they are not sufficiently stable to gassing.